Prior art spot mirror designs incorporated a variety of methods for securing and fixing the position of the mirror. Typically, two nuts were used in the prior spot mirror assemblies. One nut would serve to compress the components of the mirror assembly so the mirror could be secured in the desired pivotal position with respect to the vehicle. This nut also prevented the bolt from moving axially inward toward the center of the mirror. The second nut was then used to fasten the mirror assembly to the support mounted on the vehicle.
To prevent the mirror assembly from rotating about the axis of the bolt, some type of anti-rotation mechanism was needed. Typically, a tongue and groove formation was used between the rear plate of the mirror and the ball joint. This would prevent the mirror from spinning around the bolt causing the drain hole to rotate away from the bottom center position.
Another method utilized for preventing rotation of the mirror about the bolt was to make the counterbore hexagonal in shape in order to match the shape of the head of the bolt. By doing so, the inner portion of the joint with the counterbore would not be able to rotate relative to the axis of the bolt.